Donald Trump is again riling up his voting base with claims that the November election will be rigged against him.

“Of course there is large scale voter fraud happening on and before election day,” Trump tweeted Monday morning. “Why do Republican leaders deny what is going on? So naive!”

It’s a charge that even other Republicans have been quick to refute. Critics have called such talk potentially dangerous and detrimental to trust in the US democratic process.

“States, backed by tens of thousands of GOP and DEM volunteers, ensure integrity of electoral process,” Sen. Jeff Flake of Arizona, who is often sharply critical of Trump, tweeted Sunday. “Elections are not rigged.”

Jon Husted, the secretary of state of Ohio and the top election official in the key battleground state, also said Monday that he could assure Trump the election would not be rigged.

Trump’s most recent claims coincided with a plunge by him in the polls. Before now, Trump most recently made similar claims when his polls numbers were taking a dive in early August.

“And I’m telling you, November 8, we’d better be careful, because that election is going to be rigged,” the New York billionaire told Fox News host Sean Hannity in August. “And I hope the Republicans are watching closely or it’s going to be taken away from us.”

Multiple Republicans told Business Insider at the time that Trump’s assertion was both ludicrous and dangerous, as Trump would be the first US presidential candidate in modern times, possibly ever, to blame an election loss on voter fraud or a rigged election.

Allen Raymond, a former GOP operative who was involved in the 2002 New Hampshire Senate election phone-jamming scandal, called Trump’s continued insistence that the election would be rigged “detrimental to the Republic.”

“The idea that it’s rigged, I don’t know what he’s talking about,” he said in August. “I know someone that rigged elections. I mean, you know, the fact of the matter is Hillary Clinton doesn’t need to rig this election. Trump’s going to win Alabama and that’s it. She doesn’t have to do anything. It’s painful to watch.”